The present invention is directed to the elimination of problems encountered in the feed of hydrogen sulfide gas streams containing fixed nitrogen compounds, i.e. NH.sub.3 and HCN, to the modified Claus process for sulfur production.
For reference purposes, FIG. 1 illustrates a typical Claus sulfur plant which includes a thermal reaction stage where hydrogen sulfide is first oxidized to sulfur dioxide and water, and where a portion of the formed sulfur dioxide is combined with unconverted hydrogen sulfide to form sulfur and water. Both reactions are exothermic allowing steam generation for process needs. As much as 60% of the sulfur formed is in the thermal reaction stage.
This is followed by two or more catalytic Claus conversion stages where one mole of sulfur dioxide is combined with two moles of hydrogen sulfide to yield additional sulfur. The exothermic heat of reaction is removed between each stage to protect the catalyst employed.
Any residual sulfur which evades production is incinerated and vented to the atmosphere or the tail stream is processed for sulfur recovery, such as by the process of U.S. Pat. No. 3,752,877 issued to me.
Hydrogen sulfide for feed to the process is generated from many sources. Some which may be mentioned are the processing or refining of petroleum crudes, tar sands, bitumen and shale oil, and from the conversion of coal to gases or liquids. The hydrogen sulfide is generally formed by hydrogenation of sulfur compounds in the starting material. At the same time, ammonia (NH.sub.3) and hydrocyanic acid (HCN) may be and are often formed from nitrogen compounds present or introduced.
Ammonia and HCN are quite soluble in water, the ammonia making hydrogen sulfide more soluble. When water containing the extracted components is steam-stripped, the stripped-out gases contain H.sub.2 S, NH.sub.3, HCN, and water vapor as well as carbon dioxide.
The gas streams stripped from the absorption solutions may contain from 10 to 30 or more mole percent fixed nitrogen compounds calculated as ammonia.
It is known that Claus units as described above can operate satisfactorily with feed gas containing up to about 0.5 percent or possibly 1 percent by volume NH.sub.3. With higher concentrations of NH.sub.3, however, serious difficulties have been encountered in operation of the Claus plant. These difficulties are caused by formation of solid nitrogen-sulfur salts such as ammonium sulfate and ammonium hydrosulfate, and manifested by plugging of the catalyst beds, sulfur condensers and the drain pipes conveying sulfur from the condensers.
Hydrogen cyanide may be considered in the same light as NH.sub.3, since it is readily converted to NH.sub.3 in the sulfur plant reaction furnace by reactions such as: EQU HCN + H.sub.2 O .fwdarw. NH.sub.3 + CO (1)
ammonia is relatively stable under normal operating conditions for a modified Claus sulfur plant such as illustrated in FIG. 1. The thermal reaction zone temperature usually is in the range of 1,900.degree. to 2,300.degree.F, at which the rate of thermal decomposition of NH.sub.3 by the reaction: EQU 2 NH.sub.3 .fwdarw. N.sub.2 + 3 H.sub.2 ( 2)
is relatively low. Therefore, a major part of the NH.sub.3 passes unchanged through the thermal reaction zone at the usual operating temperatures.
In the catalytic conversion zone of the Claus plant, temperatures are usually lower than about 750.degree.F, and the catalyst is ineffective for promoting reaction (2). Since complete and absolute avoidance of sulfur trioxide formation by reactions such as: EQU 3 SO.sub.2 .fwdarw. S + 2 SO.sub.3 ( 3)
is industrially impractical, the undecomposed NH.sub.3 reacts by, e.g. equation (4), to form solid nitrogen-sulfur salts in the zones of lower temperature: EQU NH.sub.3 + SO.sub.3 + H.sub.2 O .fwdarw. NH.sub.4 HSO.sub.4 ( 4)
it has been widely experienced that conventional Claus plants, as illustrated by FIG. 1, cannot tolerate more than about 0.5 mole percent NH.sub.3 in the feed gas. A convention plant would be expected to have serious plugging problems due to formed solid nitrogen-sulfur salts within a few days, if operated on feed gas containing, for example, five percent ammonia as such or as hydrogen cyanide.
Better results have been achieved by me in dividing the H.sub.2 S feed gas into two streams -- one rich in NH.sub.3, the other essentially free of NH.sub.3, and dividing the thermal reaction stage into a cylindrical furnace having two spaced feed inlets. The ammonia rich feed, with all of the required oxygen, as air, is introduced into the first feed inlet, where a high reaction temperature was maintained by reactions such as: EQU 2 H.sub.2 S + 3 O.sub.2 .fwdarw. 2 SO.sub.2 + 2 H.sub.2 O (5) EQU 2 so.sub.2 + o.sub.2 .fwdarw. 2 so.sub.3 ( 6) EQU 2 nh.sub.3 + 3/2 o.sub.2 .fwdarw. n.sub.2 + 3 h.sub.2 o (7)
and for the reaction EQU 2 H.sub.2 S + O.sub.2 .fwdarw. 2 H.sub.2 O + 2S (8)
in this arrangement reaction flame temperatures increase in proportion to the amount of hydrogen sulfide-fixed nitrogen compound mixture fed to the first feed zone. When all of the mixture containing the fixed nitrogen compounds is added with some hydrogen sulfide essentially free of fixed nitrogen compounds, temperatures increase as the stoichiometric ratio of oxygen for conversion of essentially all of the hydrogen sulfide to sulfur dioxide is reached. At this point flame temperature reaches about 3100.degree.F. While ample for ammonia decomposition, the temperature approaches the point of refractory firebrick breakdown.
Independent of the temperature realized in the flame zone adjacent the first feed inlet, the balance of the feed was fed to the second feed inlet of the furnace creating a second reaction zone where the reaction products from the first feed are mixed with the second feed to promote reactions such as: EQU 3 H.sub.2 S + SO.sub.2 .fwdarw. 2S + 2 H.sub.2 O (9) EQU 3 h.sub.2 s + so.sub.3 .fwdarw. 4s + 3 h.sub.2 o (10)
temperature is reduced with an attendant reduction in sulfur trioxide formation.
An industrial plant operated in the manner described above only proved moderately successful. Initially, there was recovered about 96.5% of feed sulfur as salable elemental sulfur. After about three months of operation, however, plugging of the final catalytic converter and sulfur condenser was noted. Within one year sulfur recovery reduced to about 94.5%. Sulfur loss in the tail gas, unless treated, as SO.sub.2 increased from 3.5% to 5.5% some 57% over the starting rate of sulfur loss. Plugging of condensers and drain pipes occurred on a continuing basis.